A number of medically recognized techniques are utilized for cataractous lens removal based on, for example, phacoemulsification, mechanical cutting or destruction, laser treatments, water jet treatments, and so on.
The phacoemulsification method includes emulsifying, or liquefying, the cataractous lens with ultrasonic power and then removing the emulsified material out of the eye. A phacoemulsification system 5 known in the art is shown in FIG. 1. The system 5 generally includes a phacoemulsification handpiece 10 coupled to an irrigation source 30 and an aspiration (or vacuum) pump 40. The handpiece 10 includes a needle 15 at the distal tip (shown within the anterior chamber of the patient's eye 1) that is ultrasonically vibrated to emulsify the cataractous lens within the patient's eye 1. The handpiece 10 further includes an irrigation port 25 proximal to the distal tip of the needle 15, which is coupled to an irrigation source 30 via an irrigation line 35, and an aspiration port 20 at the distal tip of the needle 15, which is coupled to an aspiration pump 40 via an aspiration line 45. Concomitantly with the emulsification, fluid from the irrigation source 30, which is typically an elevated bottle of saline solution, is irrigated into the eye 1 via the irrigation line 35 and the irrigation port 25, and the irrigation fluid and emulsified cataractous lens material are aspirated from the eye 1 by the aspiration pump 40 via the aspiration port 20 and the aspiration line 45. Other medical techniques for removing cataractous lenses also typically include irrigating the eye and aspirating lens parts and other liquids. Additionally, some procedures may include irrigating the eye 1 and aspirating the irrigating fluid without concomitant destruction, alteration or removal of the lens, e.g., with ultrasonic power.
Aspiration can be achieved with a variety of different aspiration pumps 40 known in the art. The two most common types are (1) volumetric flow or positive displacement pumps (also referred to as flow-based pumps such as peristaltic or scroll pumps) and (2) vacuum-based pumps (such as venturi, diaphragm, or rotary-vane pumps). Each type has its own general advantages and disadvantages. Turning to FIG. 2, an example peristaltic flow pump 50 is illustrated. In this configuration, the aspiration line 45 is in direct contact with a rotating pump head 50 having rollers 52 around its perimeter. As the pump head 50 rotates clockwise, the rollers 52 press against the line 45 causing fluid to flow within the line 45 in the direction of the rollers 52. This is referred to as a volumetric flow pump because the pump 50 directly controls the volume or rate of fluid flow. An advantage with this type of pump 50 is that the rate of fluid flow can be easily and precisely controlled by adjusting the rotational speed of the pump head 50.
Turning to FIG. 3, an example vacuum-based pump 60 is illustrated. This type of pump indirectly controls fluid flow by controlling the vacuum within the fluidic circuit. For example, the vacuum-based pump 60 can be a pneumatic pump (e.g., a venturi pump) that creates a pressure differential in a drainage cassette reservoir 65 that causes the fluid to be sucked from the aspiration line 45 into the drainage cassette reservoir 65. Thus, instead of pushing fluid through the aspiration line 45 like the flow pump 50, the fluid is essentially pulled by vacuum through the line 45. The rate of fluid flow generated by a vacuum-based pump is generally higher than the rate of fluid flow generated by a volumetric flow based pump because the vacuum-level is generally higher; however, the control of the rate of fluid flow generally involves a different control mechanism.
As is well known, for these various surgical techniques it is necessary to maintain a stable volume of liquid in the anterior chamber of the eye, and this is accomplished by irrigating fluid into the eye at the same rate as aspirating fluid and lens material. For example, see U.S. Pat. No. 5,700,240, to Barwick et. al, filed Jan. 24, 1995 (“Barwick”) and U.S. Pat. No. 7,670,330 to Claus et. al, filed Mar. 21, 2005 (“Claus”), which are both hereby incorporated by reference in their entirety. During phacoemulsification, it is possible for the aspirating phacoemulsification handpiece 10 to become occluded. This occlusion is caused by particles blocking a lumen or tube in the needle 15 of the handpiece 10, e.g., the aspiration port 20 or irrigation port 25. In the case of volumetric flow based pumps, this blockage can result in increased vacuum (i.e. increasingly negative pressure) in the aspiration line 45 and the longer the occlusion is in place, the greater the vacuum. In contrast, with a vacuum-based pump, this blockage can result in a volumetric fluid flow drop off near the aspiration port 20. In either case, once the occlusion is cleared, a resulting rush of fluid from the anterior chamber into the aspiration line 45 can outpace the volumetric flow of new fluid into the eye 1 from the irrigation source 30.
The resulting imbalance of incoming and outgoing fluid can create an undesirable phenomenon known as post-occlusion surge or fluidic surge, in which the structure of the anterior chamber moves rapidly as fluid is replaced due to the dynamic change in the rate of fluid flow and pressure. Such post-occlusion surge events may lead to eye trauma. The most common approach to preventing or minimizing the post-occlusion surge is to quickly adjust the vacuum-level or rate of fluid flow in the aspiration line 45 and/or the ultrasonic power of the handpiece 10 upon detection of an occlusion. Many surgeons rely on their own visual observations to detect the occlusion; however, because of the unpredictable and time-sensitive nature of the problem, a reliable computer-based detection and response system is preferable.
For current systems with volumetric flow pumps 50, if an occlusion occurs, the flow rate will decrease at the aspiration port 20 and the vacuum level within the aspiration line 45 between the pump 50 and the handpiece 10 will increase. Thus, a computer-based system (not shown) can utilize a vacuum sensor 55 placed on the aspiration line 45 to detect the vacuum increase and respond accordingly (an example of such a system is described in U.S. Pat. No. 5,700,240, to Barwick et. al, filed Jan. 24, 1995 and U.S. Pat. No. 7,670,330 to Claus et. al, filed Mar. 21, 2005). For current systems with vacuum-based pumps 60, however, the vacuum level within the aspiration line 45 is tied to the vacuum power generated by the pump 60 and thus, may not be an effective indicator of whether an occlusion has occurred. Accordingly, an improved system and method for controlling the rate of fluid flow in vacuum based systems on the detection of occlusion within a fluid circuit is desirable.